The invention is most useful with high intensity sodium vapor lamps of the kind comprising a slender tubular ceramic arc tube which is generally mounted in an outer vitreous envelope or glass jacket. The ceramic envelope is made of a light-transmissive refractory oxide material resistant to sodium at high temperatures, suitably high density polycrystalline alumina or synthetic sapphire. The filling comprises sodium and usually includes mercury for improved efficiency, along with a rare gas to facilitate starting. The ends of the alumina tube are sealed by closure members affording connection to thermionic electrodes which may comprise a tungsten coil structure activated by electron emissive material. The outer envelope which encloses the ceramic arc tube is generally provided at one end with the usual base. The electrodes of the arc tube are connected to the terminals of the base, that is to shell and center contact, and the interenvelope space is usually evacuated in order to conserve heat.
The high pressure sodium vapor lamps which first appeared commercially in 1966 utilized end caps of niobium having niobium tubes extending through them into the ceramic arc tube. One niobioum tube which was used as an exhaust tube had an opening into the interior of the ceramic envelope, and was hermetically tipped and sealed off after the envelope had received its filling. The other niobium tube, sometimes known as the dummy exhaust tube, had no such opening and served merely as an inlead and electrode support. Niobium was used because it is a reasonably close match to alumina ceramic in coefficient of expansion, but it is a relatively expensive metal.
In my U.S. Pat. No. 3,882,346, Ceramic Arc Tube Mounting Structure, I describe an end seal which may be used to replace the end cap and dummy exhaust tube of niobium. It utilizes a ceramic plug sealed in the end of the arc tube and having a central perforation through which is sealed a lead wire of ceramic matching metal, suitably niobium for an alumina ceramic plug. This construction reduces the quantity of niobium used in the dummy seal about to the irreducible minimum. One object of the invention is to reduce in similar fashion the quantity of niobium used in the exhaust seal.
In lamps having a projecting exhaust tube, the sealed-off exhaust tube provides a reservoir for excess sodium mercury amalgam external to the arc tube proper. This places the excess amalgam in a location removed from the direct heat of the arc and of the electrode, and arc tube blackening as the lamp ages has a minimal effect on sodium vapor pressure and on lamp voltage. Also the use of an external reservoir facilitates close adjustment of the heat balance in the lamp, as by grit blasting a portion of the exterior of the niobium tube in order to regulate the heat loss therefrom to achieve optimum temperature for lumen output and long life. However the external reservoir construction has had the drawback that the exhaust tube must be located lowermost. This has necessitated two versions of a given lamp, a base up and a base down design, the arc tube being inverted relative to the jacket in one as against the other. If either version is used in the incorrect orientation, vibration or mechanical shock may cause a droplet of amalgam to drop out of the exhaust tube into the hotter arc tube. The resulting sudden rise in vapor pressure and the corresponding increase in lamp voltage may be severe enough to extinguish the lamp. In extreme cases, the relatively cool amalgam droplet has been known to cause thermal cracking of the arc tube when it strikes. Another object of the invention is to provide an end seal construction for the exhaust tube end of the ceramic arc tube which reduces the quantity of expensive niobium required, and yet allows the lamp to be burned in any orientation without the disadvantages or limitations previously described.